Recently, there has been a strong demand for a liquid crystal display apparatus which has a large size, a wide viewing angle and a multi-color performance. As to the viewing angle characteristic necessary for a large size screen, many definitions exist which call for a range in which the half-tone level is not reversed, a range in which the brightness, the contrast ratio, and the color tone are not varied, etc. In a multi-color display, it is necessary to enhance the respective color re-production, and it is also necessary to drastically improve the viewing angle characteristic according to the above stated definitions. As a liquid crystal display device which is compatible with a wide viewing angle and a large screen size, a combination system (IPS-TFT-LCD) has been proposed in which an IPS liquid crystal layer and a thin layer transistor (Thin layer transistor TFT) are provided (Oota et al, Proceeding of the Fifteenth International Display Research Conference (Asia Display '95; p. 707); and, a monitor system in which a display screen has a diagonal size of 13.3 inches (corresponding to a 15 inch CRT) has also been suggested (Kondo et al, SID '96 Digest No. 8.1).
However, to make practical use of an IPS-TFT-LCD with a screen larger than 17 inches, which will be the main CRT size used in the feature, it is necessary to develop a large size panel having a new structure using a new process. In the conventional techniques, no reference has been made to a method of adding a homogeneous alignment characteristic to a panel having a stepped difference structure, which is the subject matter of the present invention.
In an IPS-TFT-LCD, there is an inherent difficulty in the alignment processing of the display panel. The margin of the alignment processing is narrow in comparison with a conventional type TN (Twisted Nematic) system, particularly in a normally open type TN system (in which a low voltage produces a bright display and a high voltage produces a dark display). There are three main reasons for the narrow margin, as indicated by the following items (1)–(3).
(1) Stepped Difference Structure
In an IPS-TFT-LCD, in principle, it is necessary to arrange many long and narrow electrodes (called inter-digital electrodes) having a size in the order of several microns degree.
Accordingly, a fine stepped difference structure is formed. The size of the stepped difference is determined by the thickness of the electrodes and the shapes of the various kinds of layers which are formed on the electrodes, and, ordinarily, it is more than 0.1 micron. At the upper-most layer of these layers, a high molecular layer, such as polyimide, is formed as an alignment control layer (called an alignment layer). In the conventional mass-production technique, the surface of this alignment control layer is processed according to a rubbing processing technique so that a liquid crystal alignment capability is added. On the other hand, the rubbing cloth used during rubbing processing is constituted by binding narrow fibers having a thickness of substantially 10–30 microns degree, so that every single narrow fiber provides a sharing force in a predetermined direction to a local portion of the alignment layer, whereby a processing for adding the liquid crystal alignment capability is carried out. There is an extremely narrow fiber having a size of several microns degree, however such a narrow fiber can not be used in practice as the rubbing fiber, since some rigidity for adding a certain degree of friction force is required during the rubbing processing. Since the electrode interval in IPS system is 10–30 micron degree, which is the same as the diameter of the above stated fiber, the rubbing in the vicinity of a stepped difference is not carried out fully, with the result that the alignment is disturbed easily. The disturbance in the alignment causes a lowering of the picture quality, such as a rise in the black level and a lowering of the contrast ratio according to the above stated rise, as well as a non-homogeneity in the brightness.
As a method of solving the above-stated problems, a method has been proposed for flattening the surface of the alignment layer, however a complete flattening in this manner invites the following side effects, which result in this method not being of practical use. A first effect is a problem which is caused by a phenomenon in which the spacers used for controlling the thickness of the liquid crystal layer to a constant level are moved easily. Due to movement of the spacers, the spacer distribution becomes uneven and the liquid crystal layer thickness becomes nonhomogeneous, and, accordingly, a non-homogeneity of the brightness results. Further, during movement of the spacer, the surface of the alignment layer is damaged, which causes light leakage. Due to these points, it is necessary to have some degree of stepped difference to prevent movement of the spacers.
Further, to solve the problem concerning spacer movement, it is desirable to form a stepped difference on at least one of the pair of substrates. In a case where the TFT side is flattened, an attempt is made to form the stepped difference on the opposite substrate side. In an IPS system, to effectively employ in-plane switching, it is necessary to form many thin and narrow inter-digital electrodes, and, as a result, many stepped differences are formed on the TFT side substrate. To eliminate the stepped differences it is effective to coat the substrate with a thick organic high molecular layer, however, when a thick insulation layer is formed on the electrodes, it invites a lowering of the effective voltage which is applied to the liquid crystal. As a result, the threshold voltage becomes high, which causes a problem in that, since it is necessary to use a driver having a high withstand voltage, the consumption of electric power becomes high.
It is realistic to form the insulation layer having a proper thickness on the TFT side substrate so as to retain a proper stepped difference. As shown in FIG. 1, it is necessary to provide a ratio a/d between the liquid crystal layer thickness d and the stepped difference a of at least more than 0.02. In particular, for example, to drive a large size panel having a diagonal size of more than 18 inches, and to restrain a deterioration of the voltage waveform from a driver LSI, it is necessary to lower the resistance value of the electrodes; accordingly, the electrode necessarily should be formed so as to be thick. In a large scale panel, a stepped difference necessarily remains.
(2) Alignment Angle
In an IPS-TFT-LCD, it is necessary to establish, in principle, an initial alignment direction to set the direction in which the electrodes are to extend, or by shifting by more than a constant angle from a vertical direction relative to the above-stated direction. Herein, the electrode represents a signal wiring electrode, a common electrode and a pixel electrode in a pixel. To regulate the initial liquid crystal alignment direction by use of a rubbing method, as stated above, it is necessary to rub in a predetermined angle direction using fibers having a size of about 10–30 microns degree, with the stepped difference between the wiring being extended in a constant direction, such as the direction of the signal wiring electrode, the common electrode in the pixel, and the pixel electrode in the pixel and the end portions thereof. However, this causes a problem in that the fibers are drawn in the stepped difference direction from the design angle.
(3) Profound Degree of the Black Level
One of the characteristics of an IPS-TFT-LCD is that the profound degree of the black level (the black display) is good. Accordingly, the disturbance in the alignment comes to the fore in comparison with other systems.
In the conventional normally open type TN system, the dark level is obtained when a high voltage is applied. In this case, under the high voltage condition, almost all of the liquid crystal molecules are directed to the electric field direction, which is a direction vertical to the substrate face. With such a relationship between the liquid crystal molecules and the polarizing plate arrangement, the dark level can be obtained. Accordingly, the homogeneity of the dark level does not depend in principle on the initial alignment condition during the low voltage time. Further, the human eye recognizes the roughness of the brightness as a relative ratio and further reacts with a logarithm scale, and, accordingly, the human eye is sensitive to a fluctuation of the dark level. From the above stated view points, in the conventional normally open type TN system, in which the liquid crystal molecules are arranged compulsively in one direction under a high voltage state, there is an insensitivity to the initial alignment condition, so that it is in a better position. On the other hand, in the in-plane switching system, to display the dark level at the low voltage state or at a voltage of zero, there is a sensitivity to the disturbance of the initial alignment condition. In particular, an arrangement (called a bi-refraction mode) is formed in a homogenous arrangement, where the liquid crystal molecules are aligned in a direction parallel to each other on an upper substrate and a lower substrate; and, also, the light transmission axis of one polarizing plate is formed in parallel to the liquid crystal molecular alignment direction and that of the other polarizing plate is formed orthogonally thereto, so that for a polarization light which is incident on the liquid crystal layer, a linear polarizing light is propagated without a disturbance. This effect is profound at the dark level.
The transmission rate T of the bi-refraction mode is expressed by a following formula (1).T=To×sin2 {2θ(E)}×sin2 {π×deff×Δn/λ}  (1)
Herein, To is a coefficient and is a numerical value which is determined mainly by the transmission rate of the polarizing plate used in the liquid crystal panel, θ(E) is an angle made by the alignment direction (an effective light axis of the liquid crystal layer) of the liquid crystal molecules and the polarization light transmission axis, E is an applied electric field strength, deff is an effective thickness of the liquid crystal layer, Δn is a refraction rate aeolotropic characteristic, and λ is the wavelength of the light. Further, herein, a product of the effective thickness deff of the liquid crystal layer by the refraction rate aeolotropic characteristic Δn, namely (deff×Δn), is referred to as retardation. Further, the thickness deff of the liquid crystal layer is not the whole thickness of the liquid crystal layer, but when a voltage is applied, it indicates the thickness of the liquid crystal layer in which the alignment direction changes in practice. Because the alignment direction of the liquid crystal molecules at the vicinity of an interface of the liquid crystal layer do not change when the voltage is applied, there is an effect of an anchoring of the interface. Accordingly, when the thickness of the whole liquid crystal layer which is sandwiched by the substrates is expressed by dLC, between this thickness dLC and the thickness deff, a relationship deff<dLC is maintained, and this difference varies according to the liquid crystal material used in the liquid crystal panel and the interface contacting the liquid crystal layer, for example, the kinds of alignment layer materials being used, and it can be estimated to be, in general, 20–40 nm degree.
As clearly understood from the above stated formula (1), the component which depends on the electric field strength is sin2{2θ(E)}, so that by changing the angle θ in response to the electric field strength E, the brightness can be adjusted. To form a normally close type, during the voltage non-application time, since the polarizing plate is established to have θ=0 degree, it is sensitive to the disturbance of the initial alignment direction.
(4) Problems of the Photo-Alignment Method
As explained above, as to the problems (1) to (3), it is possible to solve these problems by use of the photo-alignment method in place of the conventional rubbing method. The photo-alignment method is classified largely into two basic types, i.e. a photo-dissolution type and a photoreaction type. In both cases, from a practical point of views, there are the following problems. To provide an improvement in the close adhesion characteristic between the alignment layer material and the substrate, it is necessary to have some degree of thickness, however, in this case, it is difficult to obtain the necessary compatibility between the photo-reaction characteristic and the transparency characteristic. In such a case, bad coloring is evident, and the utilization efficiency of the light and the picture quality are reduced. The assurance of providing a close adhesion characteristic between the alignment control layer and the substrate is important in practical use and is critical to the success or the failure of the photo-alignment.
Further, as stated above, to employ photo-alignment for a substrate having a stepped difference structure, it is necessary to give adequate consideration to the following points. In the photo-alignment method, to a whole face of the alignment layer, the light having an ability for reforming a surface, for example, ultraviolet light, is irradiated. In this case, since a reflection of the light is caused at the stepped difference portion, it is necessary to give consideration to the light path. It is desirable to have a modulated and inclined taper structure as the stepped difference. In a case when the ratio a/b of the parameter shown in FIG. 1 is less than 1 (the inclination angle is less than 45 degrees), the refection light at the taper portion will not reach the pixel portion, and, accordingly, it does not cause an alignment failure.